HAVANA (Reuters) - Colombia's FARC rebels announced on Monday a two-month unilateral ceasefire, the first truce in more than a decade, as peace negotiators met in Cuba in the latest attempt to end the five-decade war.

President Juan Manuel Santos' government, however, has said it would not cease military operations until a final peace deal was signed with the Andean country's largest rebel group and even vowed to step up the offensive.

The FARC said it would halt all offensive military operations and acts of sabotage against infrastructure beginning at midnight on Monday night and running through January 20.

"This policy decision of the FARC is a contribution made to strengthen the climate of understanding necessary so that the parties that are starting the dialogue achieve the purpose desired by all Colombians," FARC lead negotiator Ivan Marquez said as he arrived for talks in Havana.

The conflict has dragged on for nearly half a century, taking thousands of lives and displacing millions in Latin America's longest running insurgency.

Failure would mean years more of fighting and further blight on the reputation of a country eager for more foreign investment and regional clout, yet unable to resolve its most serious domestic problem.

Delegations for the government and the FARC arrived in black luxury cars at Havana's convention center where they will meet almost daily until the talks end.

The complex is located in Cubanacan, Havana's plushest neighborhood, filled with palatial houses that once belonged to the city's elite, virtually all of whom fled Cuba after the 1959 revolution that transformed the island into what is now one of the world's last communist states.

The conflict proved to be intractable in three previous peace processes, but both the Colombian government and the FARC, or Revolutionary Armed Forces of Colombia, have expressed optimism that this time might be different.

Santos wants an agreement within nine months, but the two sides face plenty of thorny issues in their five-point agenda, which will begin with rural development.

The other four points are the political and legal future of the rebels, a definitive end to the conflict, the problem of drug trafficking and compensation for war victims.

"We hope, as also hope the majority of Colombians, that the FARC shows that they think this is the moment for the force of ideas and not the force of bullets," lead government negotiator Humberto de la Calle said as he left Bogota for Havana on Sunday.

"The government wants to end the conflict as a first step to advance the construction of a stable peace and in this scenario the FARC converted into a legal political party fits," the former vice president said.

SOCIAL INEQUALITY

The conflict dates back to 1964 when the FARC emerged as a communist agrarian movement intent on overturning Colombia's long history of social inequality.

The group has been weakened by a U.S.-backed military offensive begun in 2002 that has reduced its numbers to about 8,000 and forced them into remote mountain and jungle strongholds.

But it still has the strength to launch attacks that Santos wants ended so the country can grow its economy, boosted in recent years by fast-growing oil and mining sectors.

The FARC has sustained itself by cocaine trafficking, kidnapping, ransom and "war taxes" charged within the territories it controls.

Its leaders deny involvement in the drug trade and renounced kidnappings earlier this year, but the United States and European Union consider it a terrorist organization.

Marquez, a member of the FARC's secretariat, will lead a delegation of about 30 people at the talks, which were formally begun last month in Norway.

He has said the talks are likely to take longer than Santos wants because so many complicated issues must be resolved.

In Norway, he called for ousting foreign companies fueling Colombia's oil and mining boom but de la Calle said the talks must stick to the agenda if they are to be successful.

The agreed upon topics already hold numerous potential stumbling blocks, among them land reform, decisions on which FARC leaders will be allowed to participate in politics and who must go to jail for the group's crimes and involvement in the drug trade.

The rebels have said they will ask for a ceasefire, but Santos said he will instead increase military pressure to try to force a peace agreement.

"It will not be through intransigence on our part that these talks will not have success in a reasonable time," Santos said over the weekend at the Ibero-American Summit in Cadiz, Spain.

Norway is a guarantor of the process, along with Cuba.

Officials want the talks held in the strictest possible secrecy, which is likely the reason they are in Cuba, where the government is expert at keeping information close to the vest.